Method for oil production and gas injection



Dec. 20, 1966 A. e. WEBER 3,292,703

METHOD FOR OIL PRODUCTION AND GAS INJECTION Filed Sept. 50, 1963 Alvin G. Weber INVENTOR.

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ATTORNEY Unitcd States Patent 3,292,703 METHOD FOR OIL PRODUCTION AND GAS INJECTION Alvin G. Weber, Tulsa, Okla, assignor, by mesue assignments, to Esso Production Research Company,

Houston, Tex., a corporation of Delaware Filed Sept. 30, 1963, Ser. No. 312,534

Claims. (Cl. 16642) This invention relates to a method for improving the efficiency of oil recovery from subsurface reservoirs. More particularly the invention relates to an improved method of pressure maintenance by alternate cycles of gas injection and oil production in the same wellbore.

Pressure maintenance by gas injection has been a common practice in the recovery of oil for many years. When gaseous pressure is applied tot he upper portion of an oil-bearing formation, the oil in the affected area is forced downward through the oil sand. Gradually as the pressurization continues and a greater quantity of gas is forced into the formation, a definite separation of the reservoir into a zone of gas saturation and a lower zone of oil saturation occurs. This segregation results in the formation of a gas-oil interface within a reservoir, which, as the gas cap expands and increases in volume, moves gradually downward through the horizon.

While this pressurization is effected at the upper level of a formation, oil is withdrawn from the lower level of the reservoir. Typically the gas injection is carried out at one or more separate injection wells laterally spaced from one or more production wells penetrating the same reservoir. However it has been recognized heretofore, particularly in reservoirs characterized by low structural relief, that gas pressurization and oil production can be carried out alternatively or concurrently through a single wellbore. An example of this technique is found in US. Patent No. 2,754,911.

A severe limitation upon the efficiency of such prior processes is the tendency for an early breakthrough of injected gas to occur at the production level. Ideally it was contemplated that the injected gas being much less dense than the reservoir oil, would spread horizontally within the reservoir and remain near the level of its injection. Gravity segregation, however, has been found incapable of preventing gas coning. As a result gas injected in the upper portion of a reservoir soon migrates downward in the vicinity of the wellbore because of pressure differences, and re-enters the wellbore at the production level thereby drastically reducing the rate of oil flow. Accordingly, it is an object of the present invention to overcome, or minimize the back-flow and coning of injected gases into the lower perforations during the production cycle of the process.

In accordance with the present invention alternate cycles of oil production from a lower portion of the reservoir, and gas injection into the upper level of the formation, are carried out in the same wellbore. At the end of each gas injection cycle, a slug of produced crude, LPG, or other suitable liquid is injected into the formation at or near the level of gas injection. A fluid block is thereby established which inhibits coning or backfiow of injected gas into the lower portion of the reservoir during a production cycle.

It is contemplated that this invention will be particularly useful in artificial gas drives and will improve gravity dainage in reservoirs of low structural relief where the benefits of gravity segregation would normally not be realized.

As shown in the drawing, two strings of tubing are cemented in wellbore 11 by conventional dual completion techniques. Tubing 12 is for oil production and is Patented Dec. 20, 1966 "ice therefore provided with perforations 13 only in the lower level of reservoir 14. Tubing string 15 is for gas injection and is therefore provided with perforations 16 only in the upper portion of the producing formation.

The perforated interval for gas injection should extend downward from a level near the upper boundary of the oil-bearing horizon to a depth no greater than one-third of the formation thickness, and preferably no greater than one-fourth the formation thickness. The interval perforated for oil production should extend upward from a level near the lower boundary of the horizon to a point substantially below the lowermost perforations for gas injection.

Various dual completion techniques are known and practiced in the industry. A number of these are discussed in the Petroleum Engineer, March 1961, pp. B57- 76. Any such technique for establishing separate communication with the upper and lower levels of an oilproducing formation is suitable for use in carrying out the present invention. A multiple tubingless completion such as shown in the accompanying drawing is disclosed in US. Patent No. 3075,582.

Alternate cycles of oil production and gas injection are then initiated. At the end of each gas injection cycle a slug of produced crude or other suitable liquid is injected into formation through injection string 15. The injected oil or other liquid forms a liquid block 17 which inhibits or delays the back-flow and coning of injected gas 18 into lower perforations 13 during a subsequent oil production cycle. Oil production is continued until an excessive gasoil ratio is observed, at which time the process is usually repeated.

It is sometimes unnecessary to follow each breakthrough of gas at the production level with additional gas injection at the upper level. That is, the injection of additional oil or other liquid may follow gas breakthrough, whereby the gas saturation is again displaced from the vicinity of the wellbore, permitting a new production cycle without an intervening step of gas injection.

It is also within the scope of the invention to produce oil from the lower level of the reservoir while injecting gas into the upper level at the same time. However, it is preferred to operate in a cyclic manner, in order to minimize the pressure difference between upper and lower parts of the reservoir during gas injection.

Under favorable conditions this process reduces gas production, aids gravity segregation in low relief reservoirs, and enhances ultimate recovery of oil from the reservoir. As a result, gas injection can be made more competitive with Water injection thereby alleviating the current problem of excessive gas waste in some parts of the world.

As a specific example of the invention, separate injection and production tubing strings are cemented in a wellbore which penetrates a producing formation having a permeability of 64 millidarcies (both horizontally and vertically) and a thickness of 93 feet. The perforations in the injection string extend from the upper boundary of the oil-bearing formation to a depth of 20 feet therein. The production string perforations extend from the lower boundary of the formation to a height of 43 feet therefrom which provides an interval of 30 feet between the lowermost injection perforations and the uppermost production perforations. Gas is injected through the upper perforations until the gas saturation extends about feet radially from the wellbore. About 3500 reservoir barrels of produced crude are then injected to p event gas coning or backfiow, after which the injection string is shut in at the well head, and a production cycle is begun. The solution gas-oil ratio of fluids initially produced is approximately 800 std. cubic feet per barrel. After sixteen days of production at a rate of 1000 reservoir barrels per day, no appreciable increase in the gas-oil ratio is observed. When gas breakthrough ultimately occurs, the production tubing is shut in and a new cycle of gas injection is begun.

For purposes of comparison the above example is repeated without the injection of oil to block the backflow and coning of injected gas. As a result the production cycle following gas injection is maintained at a rate of 1000 reservoir barrels per day for only about 3% days, at which time the gas-oil ratio suddenly increase to about 6000 cu. ft. per barrel. Thus the benefit obtained in accordance with the present invention is clearly demonstrated.

The above examples are based on calculations which assume a reservoir oil viscosity of 0.47 centipoise, and a reservoir oil density of 0.68 gram per cc. The injected gas has a density of 0.23 gram per cc. and a viscosity of 0.023 centipoise. The hydrocarbon-occupied porosity of the reservoir initially is 0.16. The average connate water saturation is 0.20.

Natural gas (principally methane) separated from the produced crude is usually an adequate source of gas for pressure maintenance by injection in accordance with the invention. However, makeup gas obtained from other sources is equally suitable, in the event additional volumes are desired. Other gases may be used, including ethane, nitrogen and flue gas.

The volume of gas introduced during the injection cycle is not particularly critical insofar as operability of the process is concerned. It is the usual practice to return a sufficient volume of gas to insure complete or partial maintenance of original reservoir pressure. Preferably, a suflicient volume of gas is injected to maintain a reservoir pressure at least 50 percent as great as the original pressure.

Produced crude oil is usually a preferred liquid to inject behind the gas bank, because of its ready availability and its obvious compatibility with formation fluids. However, any liquid having a viscosity which permits convenient injection into the formation, and a density not substantially greater than the reservoir crude, is suitable. Best results are obtained by choosing a liquid which is miscible with the reservoir crude, such as LPG, diesel oil, kerosene, naphtha or the like. Non-hydrocarbon solvents may also be used, including CO alcohols and ketones.

The oil slug or other liquid which follows gas injection must occupy a sufficient portion of the pore volume to block or inhibit coning and backflow of injected gas into the production interval. Usually, at least barrels of oil per foot of injection interval is required.

While various embodiments of the invention are specifically described, other modifications and variations will occur to those skilled in the art without departing from the proper scope of the invention.

What is claimed is:

1. A method of recovering oil from a subterranean reservoir penetrated by a wellbore, which comprises pressurizing the upper level of said reservoir by injecting a gas through said wellbore selectively into the upper part of the reservoir, thereafter blocking the injected gas from backflow or coning by injecting a liquid through said wellbore selectively into the upper part of the reservoir thereby displacing the injected gas from the vicinity of the wellbore, and producing oil through said wellbore from a lower part of the reservoir.

2. A method as defined by claim 1, wherein a suflicient volume of said gas is injected to provide atleast 50 per- 1 cent maintenance of original reservoir pressure.

3. A method as defined by claim 2 wherein the volume of said injected liquid is at least 10 barrels per foot of 1 the injection interval.

4. A method as defined by claim 1 wherein said gas is selected from the group consisting of natural gas, ethane, nitrogen, and flue gas.

5. A method as defined by claim 1 wherein said liquid is selected from the group consisting of produced crude, LPG, diesel oil, kerosene, naphtha, carbon dioxide, alcohols, and ketones.

6. A method as defined by claim 1 wherein said gas is non-condensa-ble at reservoir conditions.

7. A method for recovering oil from an oil-bearing reservoir penetrated by a wellbore which comprises dually completing said well to provide fluid communication with the upper level of said reservoir through a first set of perforations in a first tubing string, and separate fluid communication with the lower level of said reservoir.

through a second set of perforations in a second tubing string, pressurizing the upper level of said reservoir by injecting gas through said first tubing string and first i set of perforations, following said injected gas with the injection of a liquid to block the injected gas from backflowing or coning into the lower perforations during a subsequent oil production cycle, thereafter shutting in said J first tubing string at the well head, and producing oil from said lower level of the reservoir through said second set of perforations and said second tubing string.

8. A method as defined by claim 7 wherein said injection interval is extended from a level near the upper boundary of the formation to a depth no greated than one- 1 third the formation thickness.

9. A method as defined by claim 8, wherein the interval completed for oil production is extended from a level near the lower boundary of the formation to a point sub-. stantially below the lowermost injection level.

10. A method of recovering oil from a subterranean reservoir penetrated by a wellbore, which comprises in- I jecting a gas through said wellbore selectively into the upper part of the reservoir, thereafter injecting a liquid through said wellbore selectively into the upper part of I the reservoir thereby displacing the injected gas from i the vicinity of the wellbore, producing oil through said wellbore selectively from a lower part of the reservoir until a substantial increase in the gas-oil ratio is observed, then again injecting a liquid selectively into the upper part of the reservoir to displace gas from the vicinity of the wellbore, and again resuming the production of oil through said wellbore from a lower part of the reservoir.

References Cited by the Examiner UNITED STATES PATENTS 2,593,497 4/1952 Spearow 166-42 3,120,265 2/1964 Allen 166-45 X 3,126,961 3/1964 Craig et al. 166-40 CHARLES E. OCONNELL, Primary Examiner.

S. J. NOVOSAD, Assistant Examiner. 

1. A METHOD OF RECOVERING OIL FROM A SUBTERRANEAN RESERVOIR PENETRATED BY A WELLBORE, WHICH COMPRISES PRESSURIZING THE UPPER LEVEL OF SAID RESERVOIR BY INJECTING A GAS THROUGH SAID WELLBORE SELECTIVELY INTO THE UPPER PART OF THE RESERVOIR, THEREAFTER BLOCKING THE INJECTED GAS FROM BACKFLOW OR CONING BY INJECTING A LIQUID THROUGH SAID WELLBORE SELECTIVELY INTO THE UPPER PART OF THE RESERVOIR THEREBY DISPLACING THE INJECTED GAS FROM THE VICINITY OF THE WELLBORE, THE PRODUCING OIL THROUGH SAID WELLBORE FROM A LOWER PART OF THE RESERVOIR. 